The present invention relates to a system and method of attitude determination. More specifically, it relates to a system and method wherein the attitude of a platform may be determined based on the differences between the phase of a carrier signal received at different antenna elements on the platform.
The Global Positioning System (GPS) is an example of a current navigation system in which numerous signals are transmitted from points which are known or ascertainable by the receiver. By tracking signals from such a system, a receiver may be able to derive information such as its position, direction, or velocity.
It is also known how to calculate the attitude of a platform, from phase differences of such signals between different antenna elements on the platform, using known information about the physical arrangement of the antenna elements relative to each other. Some known systems use separate receiver channels to track each incoming signal simultaneously from different antennas. A first difference of the phase measurements from a single incoming signal obtained at a pair of antennas is formed to remove receiver clock biases and selective availability (SA) errors. SA refers to intentional degradation of some signals which are commercially available. A second difference of the phase measurements from a pair of signals is formed to remove common antenna transmission path delays. The phase difference measurements must be adjusted for the integer number of phase cycles between pairs of antennas, and this cycle ambiguity is normally determined by systematic search methods which minimize some measure of phase error. These designs are costly, introduce channel to channel biases, have stringent timing requirements, and may need to operate multiple receivers from a common clock.
In one embodiment of the present invention, the attitude of a platform is determined by time-multiplexing among antenna inputs, when the platform is stationary or the receiver is integrated with an inertial system for motion compensation. The resulting signal can be applied to each channel of a multi-channel receiver. Each channel is capable of tracking one of the incoming signals, and the phase differences for that one incoming signal are determined by digital processing, as the signal being applied to all of the channels is switched from one antenna input to another. The number of incoming signals being tracked, and consequently the number of signals for which phase differences between the antenna elements can be measured, can correspond with the number of channels--regardless of the number of antenna elements. The phase difference determinations are used to determine the attitude of the platform.
This approach minimizes the number of channels required, and alleviates channel bias, timing, and common clock problems. It is also a simpler technique because only a single difference of measurement data is required. In addition, the multiplexing may occur at a slow rate of one Hertz (Hz) or even less. The slow rate results in less noise than is possible with higher rate multiplexing, and in lower demands on the receiver's processing capabilities.
The features of the present invention which are believed to be novel are set forth below with particularity in the appended claims. The invention, together with further advantages thereof, may be understood by reference to the following description in conjunction with the accompanying drawings, which illustrate specific embodiments of the invention.